The present invention relates generally to nuclear fuel assemblies and, more particularly, to a novel nozzle arrangement employed in a fuel assembly.
The reactor core of a typical fast breeder nuclear reactor is designed to sustain a continuous sequence or chain of fission reactions, and generally contains a multiplicity of similarly constructed fuel assemblies vertically oriented in a side-by-side relation. Each fuel assembly, in turn, contains a multiplicity of thin, elongated fuel elements or pins containing stacked fuel pellets formed of radioactive material, such as uranium, thorium, or plutonium compounds for example. As is known, the heat generated by the fission reactions is transferred to a circulating coolant, such as liquid sodium for example, and subsequently transmitted to a secondary coolant, such as water, for conversion into steam for generating electrical energy.
In addition to the multitude of fuel pins, each fuel assembly usually comprises a relatively thin-walled duct tube serving as the jacket or housing for the fuel pin as well as a conduit for the passage of coolant therethrough, a shield/inlet nozzle assembly for introducing the coolant into the fuel assembly, and a handling socket at the other end of the duct tube to facilitate insertion and removal of the assembly into and from the core. Each fuel assembly must maintain its structural integrity during various stages of reactor operation including heat up, cool down, shut down, and powered operations as well as withstand the most adverse operative conditions expected during its lifetime.
The shield/inlet nozzle assembly allows coolant to enter the fuel assembly while providing neutron shielding. It structurally ties the fuel pins to the support member and normally is welded to the duct tube to provide overall structural continuity for the fuel assembly. Conventionally, the nozzle assembly contains several block sections, such as an orifice block, a neutron shield block, and a diffuser block all welded in place within the nozzle assembly. These several block elements are formed with various orifice or passage patterns to establish coolant flow requirements dictated by the specific fuel assembly being designed. Since these assemblies differ according to their function and/or location relative to the axis of the nuclear core, it can be appreciated that a variety of different orificing arrangements must be contemplated. Moreover, once a particular orifice pattern is incorporated within a specific assembly, this orifice pattern remains fixed and, since varying power conditions and temperatures are encountered during an operating cycle, optimum coolant flow during various stages of operation cannot be realized.
Another problem encountered in liquid metal cooled reactors, particularly those utilizing liquid sodium as a coolant, is their susceptibility to a phenomenon often referred to as "Flow Impedance", which is the gradual increase of pressure required to maintain a constant volume of coolant flow. This is believed to be caused by the deposition of particulates, such as silicon compounds, on the inlet end of core components and on the wall surfaces of the piping system. It is estimated that the effects of such phenomena can result in an operating penalty of $40,000,000 a year in a large commercial reactor. For a more detailed description of this phenomenon, as well as one approach for solving this problem, reference may be made to U.S. patent application, Ser. No. 554,867, filed on Nov. 25, 1983, and assigned to the saae assignee as the present invention.
Accordingly, it is a primary object of the present invention to obviate the above noted shortcomings by providing a new and useful inlet nozzle assembly for nuclear reactor fuel assemblies.
It is another object of this invention to provide in the foregoing nozzle assembly replaceable internal block elements to simplify fabrication and optimize fuel assembly design, effect nozzle standardization of core component assemblies, permit quick and easy non-destructible dismantling and replacement of assembly components, and facilitate cleaning of the several elements and components.
It is still another object of the present invention to provide in the foregoing inlet nozzle assembly a novel orifice plate assembly comprised of a plurality of differently orificed plates selectively utilized in accordance with desired coolant flow requirements for a specific fuel assembly.
It is a further object of this invention to detachably connect the above mentioned nozzle assembly to the associated fuel assembly to facilitate removal and replacement of the internal elements of the inlet nozzle assembly.
It is still a further object of the present invention to provide the foregoing nozzle assembly with means for removing particulates from the coolant stream flowing therethrough.
These and other objects, advantages, and characterizing features of the present invention will become clearly apparent from the ensuing detailed description of an illustrative embodiment thereof, taken together with the accompanying drawings wherein like reference characters denote like parts throughout the various views.